Studies will be carried out to gain an understanding of the physiologic role of placental thyrotropins in the control of thyroid function during pregnancy. A sensitive receptor assay for thyroid-stimulating hormone (TSH) will be developed. Thyroid-stimulating activity will be measured at monthly intervals during pregnancy using a more sensitive mouse bioassay and the TSH receptor assay. The results will be compared with radioimmunoassays of serum pituitary TSH, chorionic thyrotropin (hCT), chorionic gonadotropin (hCG), T4, T3, free T4 and free T3. A more sensitive radioimmunoassay will be developed for chorionic thyrotropin. The thyroid-stimulating activity of hCG will be tested in euthyroid men and in experimental animals. The effect of hypothalamic releasing factors on the secretion of hCT and hCG by placental tissue will be tested in a perifusion system.